This invention relates to production of high octane fuel from naphtha by hydrocarbon cracking to produce isomeric intermediate paraffins, and subsequent upgrading to make ethers. In particular, it relates to methods and reactor systems for cracking C.sub.7 + paraffinic and naphthenic feedstocks, such as naphthenic petroleum fractions, under selective reaction conditions to produce intermediates rich in C4-C5 isoalkanes.
There has been considerable development of processes for synthesizing alkyl tertiary-alkyl ethers as octane boosters in place of conventional lead additives in gasoline. The etherification processes for the production of methyl tertiary alkyl ethers, in particular methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) have been the focus of considerable research. It is known that isobutylene (i-butene) and other isoalkenes (branched olefins) may be reacted with methanol, ethanol, isopropanol and other lower aliphatic primary and secondary alcohols over an acidic catalyst to provide tertiary ethers. Methanol is considered the most important C.sub.1 -C.sub.4 oxygenate feedstock because of its widespread availability and low cost. Therefore, primary emphasis herein is placed on MTBE and TAME and cracking processes for making isobutylene and isoamylene reactants for etherification.
In current refining strategies naphtha reforming provides a major source of high octane gasoline containing very high aromatic levels, including benzene. In the present integrated process, the naphtha feedstock is first partially converted in a cracking reactor containing large-pore zeolite catalysts to obtain a mixture of light olefins and iso-paraffins. The iso-paraffins such as iso-butane and iso-pentane are separated from the product gas mixture and then dehydrogenated to corresponding iso-butylene and iso-amylene. The naphtha cracking/dehydrogenation process yields very high amount of light olefins, particularly iC.sub.4 - and iC.sub.5 - for downstream MTBE/TAME production. This processing sequence produces high octane gasoline components while minimizing the overall aromatic content of gasoline pool.